Composites of nickel and alumina are well known and widely used catalysts for the hydrogenation of carbon monoxide and/or carbon dioxide to produce methane-rich product gases. The process is generally referred to as methanation, and is highly exothermic in nature. One problem of great concern encountered in such methanation processes relates to the extreme sensitivity of nickel catalysts to poisoning by sulfur compounds, usually H.sub.2 S. This sensitivity has in the past necessitated the use of guard chambers to remove trace amounts of sulfur compounds from the feed gases, and/or other expensive sulfur removal processes for pretreating the feed gases. In general, in order to insure acceptable catalyst life, it is necessary to reduce the sulfur content of the feed gas to less than 1 ppm H.sub.2 S, and according to some authorities to less than 0.005 grains of sulfur per 1000 cubic feet of gas, which is less than 0.008 vppm (Catalysis, Vol. IV, Emmett ed. Reinhold Publishing Co. 1956 p.506). This problem is further aggravated by the fact that sulfur poisoning of nickel methanation catalysts has heretofore been regarded as irreversible, the sulfur-poisoned catalyst being non-regenerable. (Richardson, J. T., "SNG Catalyst Technology", Hydrocarbon Processing, December 1973, page 94; Catalysis Vol IV supra pp 504-506).
It has been firmly established that the active species of nickel for methanation is metallic nickel. The sulfur poisoning of conventional nickel methanation catalysts is attributable mainly to conversion of the active metallic nickel to inactive sulfides such as NiS. The non-regenerability of the poisoned catalysts arises because of the practical impossibility of reducing the nickel sulfide back to metallic nickel with hydrogen at temperatures below those at which sintering of the nickel occurs, i.e. below about 1500.degree. F.
In U.S. Pat. No. 2,697,078 a nickel oxide catalyst employed for hydrodesulfurization is said to be regenerable by hydrogen reduction at 900.degree. F, and it is speculated that in addition to removing deactivating coke deposits, the regeneration also effects the reduction: NiS + H.sub.2 .fwdarw. Ni + H.sub.2 S. However, as will be shown hereinafter, this reduction does not occur to any significant degree, and it must hence be concluded that the reported regeneration was due solely to the removal of deactivating coke or other deposits. It is now well established that nickel sulfide itself is very active for hydrodesulfurization.
U.S. Pat. No. 2,393,909 discloses a synthesis (Fischer-Tropsch) process utilizing a Group VIII metal catalyst, iron, cobalt and nickel being mentioned. The catalyst, after becoming deactivated by deposits of hydrocarbons, sulfur, carbon, etc. is said to be regenerable by stripping with hydrogen at undisclosed temperatures. The hydrogen is said to remove sulfur as hydrogen sulfide, and to physically or chemically strip hydrocarbonaceous materials from the catalyst. Here again, it must be presumed that any regeneration obtained was due to the removal of hydrocarbonaceous deposits and/or the hydrogenation of organic sulfur associated with such deposits, since none of the sulfides or iron, cobalt or nickel are reducible to the free metal under feasible reducing conditions. The principal and preferred catalyst disclosed for the synthesis reaction, iron, is well known to be much less sensitive to sulfur poisoning than is nickel (Catalysis, Vol. IV supra, p. 506).
In view of the practical irreducibility of nickel sulfide, and the established requirement for free metallic nickel in active methanation catalysts, the regeneration achieved herein is very surprising. The explanation is believed to be related to a unique characteristic of the present catalysts, which distinguishes them from prior art catalysts, namely their relatively low specific nickel surface area, ranging between about 5 and 50 m.sup.2 /g of Ni. Known prior art methanation catalysts exhibit a relatively high specific nickel surface area, ranging upward from about 60 m.sup.2 /g of Ni. This can only mean that a larger proportion of the nickel in the original fresh catalysts of this invention is in a catalytically inactive form, as compared to prior art catalysts.
The inactive form of nickel in the fresh catalysts utilized herein comprises at least in part, nickel aluminate, as determined by electron diffraction studies. This inactive form of nickel does not combine with sulfur, and it is postulated that during regeneration it is slowly reduced to form fresh metallic nickel, the sulfided nickel remaining in the catalyst as an inert component. This "reservior" of inactive, non-sulfided, but recoverable nickel apparently is a distingushing characteristic of the present catalysts. Prior art catalysts appear to be lacking in this reservior, which could account for the fact that they regain at best only a very transient recovery of activity upon hydrogen reduction.
It should not be concluded from the foregoing that regenerability of the present catalysts is achieved only by sacrificing activity normally associated with high specific surface areas of active metal. It has been found that the methanation reaction does not require high catalytic surface areas. For catalysts containing between about 15 and 60 weight percent of Ni, very little improvement in efficiency is obtained by providing more than about 10 m.sup.2 /g of nickel specific surface area. As used herein, the term "specific surface area" refers to surface area per gram of Ni, as determined by hydrogen chemisorption after reducing the fresh calcined catalyst in hydrogen for 16 hours at 700.degree. F, as described in J.A.C.S., 86, page 2996 (1964). The actual hydrogen chemisorption is measured by the Flow method described in J. Catalysis, 9, page 125 (1967).